Olefins often are isomerized to produce different olefins, having different double bond positions, different structures, or both, as maybe necessary for a chemical synthesis or for a process for making fuels or fuel additives. For instance, 5-ethylidene-2-norbornene ("ENB") is used as a monomer in the production of rubbery polymers. ENB is produced most conveniently by catalytically isomerizing 5-vinyl-2-norbornene ("VNB"). VNB is produced by reacting 1,3-butadiene (BD) with cyclopentadiene (CPD) in an addition reaction commonly known as a Diels-Alder reaction.
Olefin isomerization catalysts include liquid bases, such as mixtures of alkali metal hydroxides and aprotic organic solvents, mixtures of alkali metal amides and amines, and mixtures of organic alkali metal compounds and aliphatic amines. Unfortunately, the catalytic activities of the liquid bases are relatively low, and therefore large amounts of these relatively expensive catalysts must be used. In addition, recovery of the catalyst from the olefin isomerization reaction mixture is very difficult--requiring complicated separation and recovery steps, producing a substantial amount of waste that must be disposed of, and consuming a large amount of energy.
Examples of solid olefin isomerization catalysts are alkali metals supported on high surface area anhydrous supports such as activated carbon, silica gel, alumina and the like. These solid catalysts are difficult to handle because they may ignite and lose activity on contact with oxygen. Also, the isomerization performance of these catalysts is generally poor--both conversion of the feed and selectivity to the desired product are low.
Solid catalysts tend to be either pyrophoric or lacking in desirably high activity. Many of the more active solid catalysts must be separately activated or stabilized in the presence of an oxygen containing gas after the catalyst is synthesized. Isomerization catalysts are needed which do not require separate activation and which also are more resistant to reactive poisons in the olefin feed.